1. Industrial Field of the Invention
The present invention relates to a filter for use in a mobile communications apparatus, for example, a portable telephone and the like working on a plurality of frequency bands.
2. Description of the Related Art
When two communication frequency bands are close to each other, one antenna is conventionally shared by mobile communications apparatuses working on the two frequency bands. FIG. 9 is a block diagram showing a conventional system in which an antenna is shared by mobile communications apparatuses working on different frequency bands. Referring to FIG. 9, there are shown an antenna 51, antenna 51, a switch 52 constructed of a coaxial switch 52, a PIN diode and the like, a duplexer 53 on a transmitter (Tx) side, and a duplexer 54 on a receiver (Rx) side. A first terminal 53a of the duplexer 53 is connected to the antenna 51 via the switch 52, and second and third terminals 53b and 53c are respectively connected to a Tx side high-power amplifier PA1 working on a 900 MHz band and a Tx side high-power amplifier
working on a 1.8 GHz band, respectively via low-pass filters LPF1 and LPF2. Similarly, a first terminal 54a of the duplexer 54 is connected to the antenna 51 via the switch 52, and second and third terminals 54b and 54c are respectively connected to an RX side low-noise amplifier LNA 1 working on the 900 MHz band and an Rx side low-noise amplifier LNA 2 working on the 1.8 GHz band, respectively via bandpass filters BPF1 and BPF2. By operating the switch 52 to connect the antenna 51 to either the duplexer 53 or the duplexer 54, transmission and reception are performed on two frequency bands of 900 MHz and 1.8 GHz.
In the conventional arrangement, the receiver side duplexer and the transmitter side duplexer are connected to the single antenna via the switch, and since a plurality of transmitter side high-power amplifiers working on different frequency bands and a plurality of receiver side low-noise amplifiers working on different frequency bands are connected to the respective duplexers, via the respective band-pass filters, the component counts of the system increases. This arrangement makes it difficult to implement compact design in the mobile communications apparatuses.